Known cleaning processes for halogen-containing exhaust gases have been generally classified into wet and dry processes. The typical wet process using an aqueous alkali solution as an absorbing solution is an excellent method due to its large treating capacity and wide applicability. However, the wet cleaning process has drawbacks such as low cleaning efficiency, incomplete cleaning of halogen-containing exhaust gases, need for time- consuming maintenance of cleaning apparatus, danger of contamination of products by back streaming water from absorbing solution into semiconductor fabrication apparatus, etc. For these reasons, the wet processes are not used for cleaning exhaust gases from dry etching process. Therefore, the cleaning of exhaust gases from dry etching process have been preformed by a dry cleaning process utilizing physical adsorption or a dry cleaning process in which halogen-based gases are fixed by chemical reactions.
In semiconductor fabrication industries, dry etching techniques have been used for selectively removing parts of silicon dioxide thin films, polysilicon thin films, tungsten thin films or aluminum thin films in predetermined patterns. The etching gases for dry etching are appropriately selected depending on natures of thin films to be etched, and are required to have a high reactivity with thin films and produce reaction products readily volatilizable from thin films. To meet these requirements, halogen-based gases have been widely used as the etching gases, so that exhaust gases from dry etching process inevitably contain various kinds of halogen-based gases according to kinds of halogen-based etching gases used, kinds of thin films to be etched and etching conditions. Therefore, the treatment of exhaust gases requires dry cleaning agents and dry cleaning techniques according to kinds and properties of individual halogen-based gases contained in exhaust gases. The halogen-based gases contained in exhaust gases from dry etching process are generally classified into fluorine-based gases and chlorine-based gases. The fluorine-based gases include fluorine gas, and the chlorine-based gases include chlorine gas. Differences in chemical properties and reactivity between fluorine gas and chlorine gas are important in designing dry cleaning agents and dry cleaning processes.
In dry etching processes, in general, a halogen-based gas is fed into a chamber during etching, and an inert gas is fed after the etching. As a result, an exhaust gas containing the halogen-based gas and an inert gas containing a little or no halogen-based gas enter repeatedly into an exhaust gas cleaning apparatus.
Hitherto, in dry cleaning of fluorine-containing exhaust gases, a cleaning agent utilizing physical adsorptivity of activated carbons has been used. Such a cleaning agent can be produced at a low cost, and can remove a relatively large amount of fluorine gas and fluorine compound gases with a small amount of use. However, fluorine gas contained in exhaust gases, in some cases, explosively reacts with activated carbon. In fact, there have been reported many explosion accidents due to explosive reaction.
To avoid the above problems, a cleaning agent which is composed mainly of a metal hydroxide to utilize its chemical reactivity with fluorine has been employed for dry-cleaning fluorine-containing exhaust gases. For example, Japanese Patent Application Laid-Open No. 9-99216 teaches a cleaning agent for acidic gases, which is composed mainly of strontium hydroxide. The strontium hydroxide-based cleaning agent for acidic gases can be produced at low cost and can remove a relatively large amount of fluorine gas and various fluorine compound gases with a small amount of use. In addition, unlike activated carbon, the cleaning agent can be safely used without any risk of explosive reaction.
Contrary to the dry cleaning of fluorine-containing exhaust gases, in the dry cleaning of chlorine-containing exhaust gases, a cleaning agent utilizing physical adsorptivity of activated carbon is usable without any risk of explosive reaction, thereby enabling production of the cleaning agent at low cost.
In general, activated carbon, when used as a cleaning agent for halogen-containing exhaust gases, shows a relatively large cleaning capability in treatment of continuously flowing exhaust gases. However, the cleaning capability of activated carbon is disadvantageously decreased in treating alternative flow of an exhaust gas containing halogen-based gas and a gas (purge gas) containing no halogen-based gas as experienced in the treatment of exhaust gases from etching process of semiconductor production. This is because the halogen-based gas physically adsorbed on the activated carbon is gradually desorbed by long-term flowing of a gas containing no halogen-based gas. To eliminate this problem, various studies have been made to enhance the cleaning capability of activated carbon by adherently adding various chemical agents to activated carbon and fixing adsorbed halogen-based gas to activated carbon by chemical reaction between the chemical agents and the adsorbed halogen-based gas, thereby preventing the desorption of adsorbed halogen-based gas. For example, Japanese Patent Application Laid-Open No. 58-122025 proposes a method to adherently add alkali metal halides to activated carbon, and Japanese Patent Application Laid-Open No. 4-210236 proposes a method to adherently add alkali metal aluminate or tetraalkylammonium aluminate to activated carbon. However, any of these methods fail to exhibit satisfactory effects.
Also, various cleaning agents using no activated carbon and capable of fixing halogen-based gas by chemical reaction have been developed. For example, Japanese Patent Application Laid-Open No. 9-234337 proposes a cleaning agent prepared by adherently adding sodium formate to a metal oxide composed mainly of copper oxide and manganese oxide. The proposed cleaning agent is very excellent since it can completely fix halogen-based gases, but requires high production cost and is relatively low in cleaning capability as compared to activated carbon-based cleaning agents.
Further, a cleaning agent composed mainly of strontium hydroxide and tri-iron tetroxide (Japanese Patent Application Laid-Open No. 7-308538) shows high cleaning capability in treating exhaust gases containing halogen-based gas in high content. However, when the content of halogen-based gas is low, the cleaning capability is inferior to that of the cleaning agent prepared by adherently adding sodium formate to a metal oxide composed mainly of copper oxide and manganese oxide (Japanese Patent Application Laid-Open No. 9-234337). In addition, the cleaning agent loses water by evaporation during the use, resulting in decrease of the cleaning capability due to drying.